Fix compiling idl_parser.cpp against certain STLs

[reshilkin]

We use patched libc++ which uses raw pointers as std::string iterators.
Compiling flatbuffers against our libc++ results in the following ambiguous call:

flatbuffers/src/idl_parser.cpp:2103:16: error: call to member function 'insert' is ambiguous
    attribute_.insert(0, 1, sign);
    ~~~~~~~~~~~^~~~~~
libcxx/include/string:1153:19: note: candidate function
    basic_string& insert(size_type __pos, size_type __n, value_type __c);
                  ^
libcxx/include/string:1156:19: note: candidate function
    iterator      insert(const_iterator __pos, size_type __n, value_type __c);

This PR adds explicit casts to help the compiler.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[reshilkin]

@dbaileychess Could you, please, take a look?